Characteristics of the entire series elastic component and of tendinous structures separately (tendon and aponeurosis) were compared for rat EDL muscle-tendon complex during isometric contractions, to study the contribution of tendinous structures to series elastic component characteristics. Compliance of series elastic component was measured using quick length decreases during the force plateau of isometric contractions. Lengths of tendinous structures were measured using macro-photographs during passive and active muscle conditions. Length data obtained from aponeurosis showed inconsistency with respect to elastic behaviour in two ways: the difference of aponeurosis length in active muscle at short length and at optimum length exceeded the extension of series elastic component for the same force range. Furthermore, aponeurosis in passive muscle at optimum length was considerably longer than in active muscle at short length, despite the fact that muscle force in the former condition is smaller than in the latter. It is concluded that aponeurosis length does not depend exclusively on force but is also muscle length-dependent. This muscle length dependence was not found for tendon of EDL. Additional experiments showed that series elastic component compliance does not depend on muscle length. It is concluded that muscle length-dependent changes of aponeurosis length-force characteristics involve shifts of its force length curve to other aponeurosis lengths.